Active Galactic Nucleus (AGN) variability can be used to study the physics of the region in the vicinity of the central black hole. In this paper, we investigated intra-night optical variability of AGN in the COSMOS field in order to understand the AGN instability at the smallest scale. Observations were performed using the KMTNet on three separate nights for 2.5 to 5 hours at a cadence of 20 to 30 min. We find that the observation enables the detection of short-term variability as small as ∼ 0.02 and 0.1 mag for R ∼ 18 and 20 mag sources, respectively. Using four selection methods (X-rays, mid-infrared, radio, and matching with SDSS quasars), 394 AGN are detected in the 4 deg2 field of view. After differential photometry and X2−test, we classify intra-night variable AGN. The fraction of variable AGN (0–8%) is statistically consistent with a null result. Eight out of 394 AGN are found to be intra-night variable in two filters or two nights with a variability level of 0.1 mag, suggesting that they are strong candidates for intra-night variable AGN. Still they represent a small population (2%). There is no sub-category of AGN that shows a statistically significant intra-night variability.

We present a BV I optical photometric study of the old open cluster Ruprecht 6 using the data obtained with the SMARTS 1.0 m telescope at the CTIO, Chile. Its color-magnitude diagrams show the clear existence of the main-sequence stars, whose turn-o point is located around V  18:45 mag and B - V  0:85 mag. Three red clump (RC) stars are identied at V = 16:00 mag, I = 14:41 mag and B - V = 1:35 mag. From the mean Ks-band magnitude of RC stars (Ks = 12:39  0:21 mag) in Ruprecht 6 from 2MASS photometry and the known absolute magnitudes of the RC stars (MKS = - :595  0:025 mag), we obtain the distance modulus to Ruprecht 6 of (m -M)0 = 13:84  0:21 mag (d = 5:86  0:60 kpc). From the (J - Ks) and (B - V ) colors of the RC stars, comparison of the (B - V ) and (V - I) colors of the bright stars in Ruprecht 6 with those of the intrinsic colors of dwarf and giant stars, and the PARSEC isochrone ttings, we derive the reddening values of E(B - V ) = 0:42 mag and E(V - I) = 0:60 mag. Using the PARSEC isochrone ttings onto the color-magnitude diagrams, we estimate the age and metallicity to be: log(t) = 9:50  0:10 (t = 3:16  0:82 Gyr) and [Fe/H] = -0:42  0:04 dex. We present the Galactocentric radial metallicity gradient analysis for old (age > 1 Gyr) open clusters of the Dias et al. catalog, which likely follow a single relation of [Fe/H] = (-0:034  0:007)RGC + (0:190  0:080) (rms = 0.201) for the whole radial range or a dual relation of [Fe/H] = (-0:077  0:017)RGC + (0:609  0:161) (rms = 0.152) and constant ([Fe/H]  -0:3 dex) value, inside and outside of RGC  12 kpc, respectively. The metallicity and Galactocentric radius (13:28  0:54 kpc) of Ruprecht 6 obtained in this study seem to be consistent with both of the relations.